Bring the crab claws to a boil in salted water. Remove from the heat and let sit for 1-2 minutes, then drain.
Peel the onion and finely chop. Rinse and trim the scallions and finely chop the white part. Cut the green stems into finger-long pieces.
Heat the butter in a pan. Cook the onion and chopped scallions for 2-3 minutes while stirring. Add the crab claws and briefly cook. Stir the curry into the onions and add the green scallion pieces. Pour in the lobster stock and whipped cream and cook for 2-3 minutes.
Season with fish sauce and serve in bowls.